NO and NH3 Adsorption on Fe-SSZ-13: Insight Using DFT Calculations

NO and NH₃ Adsorption on Fe-SSZ-13: Insight Using DFT Calculations

E. Anderst, R. Zhang, F. Gao, J. Szanyi and J.-S. McEwen

The emission of hazardous NO_x gases from lean-burn diesel engines is a continual concern for the automotive and transportation sectors of developed nations, and as emission standards continue to tighten, the need for more effective and efficient methods of removing NO_x increases. To address this concern, the catalysis community has been exploring methods to reduce NO_x under the highly oxidizing conditions encountered in lean burn engines. Currently, the predominant method employed to remove NO_x is selective catalytic reduction (SCR) using ammonia over Fe and Cu exchanged zeolite catalysts. To design systems that better facilitate the removal of NO_x gases, mechanistic insight into the behavior of these zeolites is crucial. One study involving Fe-ZSM-5, proposed Fe²⁺-O⁺H-N=O as an intermediate complex, which alludes towards fundamentally different mechanistic behavior for NO adsorption between Fe and Cu-containing catalysts [1,2]. To investigate this, our study explored NO adsorption on an –OH stabilized Fe-SSZ-13 zeolite catalyst.  Density Functional Theory (DFT) calculations were conducted to elucidate the most likely reaction complexes produced upon adsorption at the active sites of the six- or eight-membered ring (6MR or 8MR) which makeup the zeolite framework. We examined both Fe²⁺ (as [Fe²⁺(OH)^-]⁺ and Fe³⁺ (as [Fe³⁺(OH)^-]²⁺) as the active sites.
            Based on our results, initially, the most energetically favorable site is found within the 6MR, however, with the addition of adsorbates, we found sites within the 8MR to be more favorable. This behavior is consistent with our previous work involving Cu-SSZ-13 [3]. Our results show that, for Fe²⁺, the most stable configuration occurs within the 8MR, where NO is co-adsorbed with OH on Fe. Likewise, for Fe³⁺, we found that the lowest energy configuration was achieved when NO adsorbed directly onto Fe within the 8MR without interaction with the OH ligand. Our findings cast doubts on the intermediate species (Fe²⁺-O⁺H-N=O) proposed by Boubnov et al. [1] and, consequently, their conclusion that Fe and Cu exchanged catalysts exhibit different behavior upon NO adsorption [1,2]. To further validate our results and provide experimentally relevant references, computational X-ray emission spectroscopy (XES), X-ray Adsorption Near Edge Structure (XANES) spectra and Infrared Spectroscopy will be produced.

[1] A. Boubnov, H. W. P. Carvalho, D. E. Doronkin, T. Günter, E. Gallo, A. J. Atkins, C. R. Jacob and J.-D. Grunwaldt, J. Am. Chem. Soc., 2014, 136, 13006

[2] T. Günter, H.W.P. Carvalho, D. E. Doronkin, T. Sheppard, P. Glatzel, A. J. Atkins, J. Rudolph, C. R. Jacob, M. Casapu, J.-D. Grunwaldt, Chem. Comm. 2015, 51, 9227.  

[3] R. Zhang, J.-S. McEwen, M. Kollár, F. Gao, Y. Wang, J. Szanyi, C.H.F. Peden, ACS Catal., 2014, 4., 4093.